The use of chlorine or bromine for water purification requires transportation of large volumes of highly toxic material (chlorine and bromine) over large distances from factory to municipality. This transportation is expensive, results in high risk of toxic spills by railway accidents, is costly, and is a security risk. Furthermore it is well known that the use of chlorine and bromine can interact with various organic and inorganic substances in the water being purified to generate carcinogens and other toxic substances. Indeed, chlorine purification is being phased out in many parts of the world in favor of ozonation.
Ozone (O3) or triatomic oxygen is a highly reactive species, which is second only to Fluorine as an oxidative species. Ozone is unstable and will revert to diatomic oxygen (O2) rapidly, with a half-life of 20 to 100 minutes (or average of 30 minutes) that decreases with an increase in temperature or humidity. Ozone can be created by the conversion of 3O2→2O3 with the addition of energy (in the form of electrons or electromagnetic radiation). This reaction is exothermic and gives off energy in the form of light and heat. Ozone can also be generated by the use of an electric discharge in gas and in liquid by using an electrolytic process.
O3 is a powerful oxidizing agent, which is used commercially for cleaning, disinfecting, deoderizing etc. It is used in applications such as air and water purification and can oxidize many organic compounds. Water purification applications include disinfection, oxidation of heavy metals, control of tastes and odors, improvement of color, breakdown of detergents, pesticides and other organic compounds, ammonia and nitrogen derivatives. Because O3 is so reactive, it is both hazardous and uneconomical to generate at any other place than the point of use. At high levels, ozone is both irritating and toxic.
The use of oxygen has been shown to be as beneficial or better than traditional water disinfectants such as chlorine and bromine, however the electrochemical potential needed to produce oxygen generates an electrical field that enhances the destruction of cellular walls and protective coatings of microorganisms such as bacteria, viruses, cysts, spores, as well as biofilms comprised of microorganisms such as bacteria, viruses, cysts, and spores. Additional benefits of using electronically produced oxygen species for water purification is that oxygen species can be produced on site and in the media to be purified. There are several advantages to producing oxygen on-site. Use of an electric field in the water or other fluid to be purified enhances bacterial kill. Ozone is generated for water purification by passing air over short wavelength ultraviolet light source (such as xenon, argon, mercury or other arc lamp) which generates ozone from the oxygen in the air, which in turn is bubbled through the water to be purified.
Ozone production is a widely used on-site method to purify water but excessive ozone reacts with bromine in the water generating highly carcinogenic bromates. This is bringing about an international water crisis as governments are setting bromate standards which are high, and likely to be unachievable. It may be important to control the amount of ozone delivered to water in order to avoid production of bromates. Another problem with ozone generation is that ozone rapidly recombines with itself to become oxygen again thereby requiring that significantly higher ozone levels be produced to get the required dissolved residual concentration necessary for disinfection of the water. Another method to kill microorganisms in water is to pass water directly over a short wavelength ultraviolet source. This may also be done on-site but may be inefficient because a long residence time is required over the light source to fully disinfect the water.
The use of electrochemical cells to produce oxygen, hydrogen and chlorine is a well established technology. These cells typically use graphite electrodes which significantly erode through attack at the oxygen generating electrode. Previous work has shown that polycrystalline and nanocrystalline boron doped diamond has a significantly lower erosion rate than graphite but eventually fails due to attack at the grain boundaries of the polycrystalline diamond. At such junctions, both sp2 graphitic diamond and other impurities, such as boron, will preferentially accumulate. Grain boundaries will have altered electrochemical properties which impede normal conductivity and reduce the efficiency of the electrodes. The lower resistivity of sp2 diamond and higher current density of boron at regions of accumulation result in preferential attack of these regions. Further, in high grade polycrystalline, high dislocation levels and crystal defects concentrate regions of boron that are susceptible to erosion. Reports of erosion of polycrystalline diamond occurs at current densities of only 0.5 to 2.5 amps per square centimeter. Such electrodes have significantly longer lifetimes than prior materials, but undergo catastrophic failure during operation.